The present invention is directed to an improved tin and tin alloy plating method that inhibits whisker formation. More specifically, the method of the present invention is directed to an improved tin and tin alloy plating method that inhibits whisker formation by an etching method and employing a copper metal coating.
Metal filaments, usually called whiskers, sometimes grow spontaneously from tin, zinc, or cadmium coatings within a period after production that may vary from days to years at a rate, which may reach 1 mm/month but is usually much less. Commonly, the growths are in the form of straight or kinked single crystals of 1-2 microns (μm) in diameter and grow from their bases, but other forms have been observed.
The growths do not affect the protective quality of the coatings, however, experience has shown that whiskers are fully capable of producing damaging short circuits in electronic equipment. Although most whiskers have diameters of only 1 to 2 μm, some have been found to carry a current of 10 mA (milliamperes) before burning out. The number of instances of trouble reported or suspected has increased over the years. More knowledge observation now identifies whiskers as a cause of failure when previously their presence might not have been suspected, but causes for a real increase are closer spacings between conductors and increased use of bright tin coatings.
Early reports of failures of electronic equipment were concerned mainly with internal faults in components such as relays but instances now occur of bridging the gaps on printed circuit and wiring boards, sometimes by whiskers falling from components but also by growths from the circuit itself. This trouble has often been associated with bright deposits but growth from chemical replacement coating of tin has also been reported; their limited thickness probably makes them vulnerable. Experience has not pointed to any environmental factors as especially influential although whiskers are seen more often on components that are sealed up and undisturbed, including those kept in storage, probably because there is a lesser chance of their removal by mechanical detachment.
An example of one such electronic component that has been subject to whisker formation is the lead frame. Devices such as integrated circuits are mechanically and electrically connected to larger assemblies via lead frames. The integrated circuit or other device is mechanically mounted on the lead frame, and the device is then electrically connected to the leads on the lead frame. The lead frame is then electrically and mechanically connected to a larger assembly. After the device is mounted on the lead frame, the device is encapsulated for protection. The process for mounting and electrically interconnecting the device to the lead frame and connecting the lead frame to a larger assembly includes steps for device attachment, cure, wirebonding, encapsulation, trim-and-forming, and soldering. Some of these steps subject the metal lead frame to mechanical stress and strain, which is believed to result in the undesirable formation of whiskers. For example, when the leads of the lead frame are formed according to the industrial standard, the forming angle is from 82 to 90 degrees and the forming radius is 250 μm.
Lead frames have been formed from a variety of materials. Lead frame materials are selected for their mechanical strength, conductivity, machinability, formability, corrosion resistance, wirebondability, solderability, and thermal expansion. Although gold and palladium have the desired characteristics, the cost of these materials makes their use prohibitive for many applications. Copper and copper alloys also have many advantageous properties that make them suited for this application. A number of different copper alloys may be used including alloy 151 (99.9 wt. % copper/0.1 wt. % zirconium); alloy 194 (97.5 wt. % copper/2.35 wt. % iron/0.03 wt. % phosphorous/0.12 wt. % zinc); and alloy 7025 (96.2 wt. % copper/3.0 wt. % nickel/0.65 wt. % silicon/0.15 wt. % magnesium). However, corrosion of copper in air and the difficulty of forming suitable soldered connections to copper create a need to use coated copper lead frames. Coating of lead frames provides corrosion protection and provides suitable solderable surfaces. An example of another suitable metal alloy for a lead frame is an iron-nickel alloy such as alloy 42 (58 wt. % iron and 42 wt. % nickel). However, corrosion of this metal alloy in air also precludes the use of the uncoated alloy as a lead frame.
A suitable coating for such metal lead frames is tin or a tin alloy. However, tin or a tin alloy plated on the lead frame may form whiskers. Whisker formation is especially noticeable at points of stress and strain in the lead frames. Early literature (see S. C. Britton, Trans. Inst. Metal Finishings, 52, 95 (1974)) alleged that copper undercoats may reduce tin whiskering, however, recent studies (see Keith Whitlaw & Jeff Crosby, Proceedings—AESF SUR/FIN Annual International Technical conference, (2002) have clearly shown that copper undercoats may accelerate whisker growth substantially. In some cases tin or tin alloy deposits, which exhibit low tendency to formation of whiskers in the absence of copper undercoat, may rapidly produce high population densities of long tin whiskers when a copper undercoat is applied to a substrate. Workers in the art attribute whisker formation due to copper underlayers to copper crystal orientation of predominantly 111 (Miller Index). Predominantly means that 60% or greater of the copper crystal orientation at the deposit surface has a Miller Index of 111. Accordingly, there is a need for a method to prevent or at least reduce whisker formation on tin or tin alloy coated substrates.